The left part of the calculator is for number entry. The top half represents the Y element of the stack, a number already entered. The bottom half represents the X element and is the place where new numbers are entered.
There are eight places where you can enter digits or characters: long, UInt32, Fixed, Octal, Hi-Double, Fract, Hex, and OSType. These types should be familiar to Macintosh programmers. Except for Hi-Double all of them are 32-bit values. Hi-Double means the high 32 bits of an 8-byte double. These are the most significant bits in a double so it is interesting to see how floating point numbers are represented. (Note that the lower half of the double is always left as zero so there is a loss of accuracy.) Whichever edit field is active controls the numeric format of the stack.
